Time-division multiplex (TDM) circuit-switching techniques have been in widespread commercial use for quite some time. Central to typical prior-art TDM arrangements is the notion of a "frame" divided into a predetermined number of time slots. The frame has a fixed, predetermined duration, and individual duplicate frames follow each other in sequential succession. Thus each time slot recurs at a fixed frequency, or rate, referred to herein as the "frame rate". For example, if the frame has a duration of 125 .mu.sec, each time slot recurs at a rate of 1/(125.times.10.sup.-6) sec=8 KHz. Each communicating frame is assigned to one or more time slots and, when the time slot(s) occur, the channel is enabled to place data on and/or remove data from the TDM medium (e.g. a communications link or a switching fabric). The traffic of the different communicating channels is thereby interleaved on the TDM medium. If a plurality of non-adjacent time slots within a single frame is assigned to a channel, that channel's traffic is also interleaved inside each frame with the traffic of other channels.
In recent years, standards have been developed for the transport of broadband communications. Among these are the Synchronous Optical Network (SONET) and the similar Synchronous Digital Hierarchy (SDH). The expected growth in synchronous transport facilities based on SONET and SDH supports a need for more efficient synchronous switch fabric architectures. The modular byte-interleaved structure of SONET is based on Synchronous Transport Signal level 1, or STS-1, format, in which overhead plus payload results in a rate of 51.840 Mb/s. The STS-1 frame consists of 90 columns by 9 rows of bytes, or 810 bytes, with a frame rate of 125 .mu.s. The first three columns in the frame are devoted to transport overhead (TOH), while the remaining 87 columns carry the payload, including one column devoted to path overhead (POH). 87 columns of payload constitute a Synchronous Payload Envelope (SPE). However, an SPE can cross frame boundaries, and is allowed to float anywhere within the payload-carrying portion of one or more contiguous frames to accommodate the semi-synchronous nature of the transport facilities. For switching of rates below the STS-1 rate, a switch assumes that the path overhead has been aligned with the first column following transport overhead.
Super STS-1 signals (STS-N) are formed by byte-multiplexing the N constituent STS-1 signals, with the resultant bandwidth being N times that of the STS-1 rate. Conversely, sub STS-1 signals are transported in Virtual Tributaries (VTs), of which four sizes are defined at present, namely VT1.5 (1.728 Mb/s), VT2 (2.304 Mb/s), VT3 (3.456 Mb/s) and VT6 (6.912 M/b/s). To accommodate mixes of VTs, the VT-structured STS-1 SPE is divided into 7 VT groups, with each group occupying 12 columns of the 9-row frame structure; 2 columns remain unused and are referred to as STUFF columns. A VT group may contain 4 VT1.5s, 3 VT2s, 2 VT3s, or 1 VT 6. Both the super STS and sub STS signals retain the frame rate of 125 .mu.s.
FIG. 2 shows a 3-dimensional representation of an STS-12 frame as an illustrative example. There are 12 vertical planes which represent the 12 STS-1s, each composed of 90 columns and 9 rows, for a total of 9720 bytes. Vertical columns may be grouped to form Virtual Tributaries (VTs), as shown by the four regularly-spaced columns representing a VT2 in position #3. While a VT2 requires 4 regularly-spaced columns, as shown, a VT1.5 requires 3 regularly-spaced columns, a VT3 requires 6 regularly-spaced columns, and a VT6 requires 12 regularly-spaced columns. Finally, a DS-0, corresponding to a 64 kilobits-per-second rate, appears as a single byte within one row and column. There are a maximum of 774 DS-0s per STS-1, some of which may be used for additional overhead functions; 756 DS-0s are available for traffic transport.
The three component sub-rates of an STS-N frame--STS-1, VT, and DS-0--may be switched independently by three separate switching fabrics, each dedicated to switching one of the sub-rates. But this is inefficient in the amount of equipment used: it requires demultiplexers at the inputs to the switching fabrics to separate the sub-rates, a separate switching fabric for each sub-rate, and multiplexers at the outputs from the switching fabrics to combine the switched sub-rates back into STS-N frames. The use of a single switching fabric for all sub-rates is therefore preferable.
Given a switching fabric capable of switching multiple rates within an STS-N format, one is faced with the problem of efficiently setting up multirate calls through such a fabric. One approach is to treat a call of any given bandwidth as multiple DS-0 calls. Although this is a flexible approach, the disadvantage is that a path-hunt and a path-setup must be performed individually for each DS-0 call. For example, a single STS-1 call would require as many as 810 individual path hunts and control-memory-setups. This is inefficient both in terms of the amount of time required for the path hunting and the number of control communications required to set up the individual paths. There is an associated need for switching elements that are adapted for efficient multirate application.